SIG J3 Cub under way - RC Groups
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Jul 25, 2006, 05:57 AM
Registered User
Build Log

SIG J3 Cub under way

Got moving on my cub again... been almost a year since I last touched it. anyone familiar with the 40 scale sig kit that has suggestions on strengthening the wing center section and where the wing halves connect... Your input will be appriceated. It just seems weak to me structure wise. I will still fiber glass the center of the plane and expoxy. 7/28 (MY Birthday-28 and I can't think of a better day to work on my cub) Slight delay on center section while I wait on hardwood order from local hobby shop. Threw in a pic of me and some friends at Kesler AFB while cleaning up after HUR Katrina. I'm the guy off duty in my civilian cloths... almost (The shirt is for my battle dress)
Last edited by chuff02; Jul 28, 2006 at 06:00 AM.
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Jul 28, 2006, 11:32 AM
Victim of gravity
Looks like the 1/6 scale Cub? There are many threads located in the Electric Scale forum. If it is, it will be fine structure wise, but follow the instructions and fly it with the struts which are totally functional. NOT meant for hurricane winds!!!
Jul 30, 2006, 10:35 AM
DPlumpe's Avatar
The center section looks weak because you cut off the spars even with the ribs at the dihedral breaks. The manual says to leave these 2-1/4" long. This lets them extend into the center section ribs and the rear spars then tie together at the center. If I were you, I'd try to extend your spars and add 3/32" hard balsa doublers, maybe 1.5" long, to the front & rear of each of the new joints.

About fiberglassing the center joints, I wouldn't use much - maybe a 1" wide strip of 3/4 or 1oz cloth at each joint - and that only if you cover with plastic film. Mine's covered with silk & dope and I didn't use any other reinforcement.

I'd also remove the diagonal ribs - they're not needed and just add weight. The covering material and struts keep the wing plenty strong & rigid. I also recommend making the strut lengths adjustable, sort of like they show on the plans, but using clevises at the tops of the struts. This lets you tweak for just a little washout.

To keep constant wing-cubed loading, a 1/6-scale Cub should weigh 5.09 lbs. Because of Reynolds number, you need to come in somewhat under that weight to fly like the full scale Cub. With an OS .26 four-stroke, mine weighs 3.75 lbs and flies great. Will stay in the air at 1/3 throttle, cruises scale-like on 1/2, and full throttle gives plenty of power for takeoff or getting out of trouble, even when I put it on floats for lake flying.

In 120 flights, I've pranged mine 3 times - once I did a spin too low over the lake, once flew into a tree, once "landed" blind behind a tree line when the engine quit. Only damage was some small holes poked in the silk from tree limbs - never any structural damage.

Keep working on that Cub - it's a GREAT flyer! Does the world's slowest snap-roll!

-Dave Plumpe
Jul 31, 2006, 09:56 AM
Registered User
Thanks for the great advice. I extended the doublers. The wing does have more flex than previous models I've built. Can see how the struts are functional. Will post more pics shortly. Will also think about removing the bracing. I may wait until I weigh the model. My next decision with the wing is 2 servos, or 1 center with bell cranks to run the ailerons. I can see how balsa pushrods and 1 servo can be lighter while 2 micro servos can be easier. Any advice.
Jul 31, 2006, 10:14 AM
Registered User


Here are some more photos.
Last edited by chuff02; Jul 31, 2006 at 10:19 AM.
Jul 31, 2006, 01:11 PM
Victim of gravity
Forget the bellcranks, and go for dual aileron servos I'd say. The clevises on the wing struts work well too....
Jul 31, 2006, 10:00 PM
Registered User
mariuslj I already started on bellcrank assembly. After some thought, I decided it would actually be faster to mount the bellcranks instead of build servo boxes for each aileron. Also I've always put seperate servos for each surface... wanted to do something new.
Jul 31, 2006, 11:48 PM
Victim of gravity

What are you going to use for propulsion?

Aug 01, 2006, 03:17 AM
Registered User


Well, I've been thinking about that. I have yet to dive into the world of Electric flight. I have 2 engines already. I have a Supertiger .51, and an OS .40. The OS .40 is still in the box and am leaning towards it. Have thought about something smaller. With either one some muffler deflection will be necessary for scale looks. Any recomendations? If cost is comprable to the Gas engine I may do the electric thing. Just not sure what all is involved.

After I finish the cub I planning to scratch out a control line plane just to test out and evaluate electric flight, then maybee X-fer the engine to a small park flyer of foam 3d type plane. I have seen electric planes at the field and have been impressed. I started as a kid building control line planes, and still have a habit of building heavy (thus the cross bracing) as a bala and tissue covered plane doesn't hold together too well unless you do some extra bracing. I fight the urge to use more epoxy, and add additional bracing everytime I pick up the knife. My intro to RC planes was a cheap 3 channel foam plane my parents bought for me more than 15 yrs ago, I took the electronics out of it and put them in a prior control line sopwith camel. That was the 1st RC plane I ever built, gusty winds killed it, and since then I've had the urge to build up plans.
Last edited by chuff02; Aug 01, 2006 at 03:32 AM.
Aug 01, 2006, 06:39 AM
Victim of gravity

The .40 will do just fine, even a little much. A friend of mine flew his very un-scale like with an OS .46. Ever seen a Cub knife edge from one side of the field to the other?!! The .26 FS mentioned by Dave seems about right, and saves the cowl hacking too.

Wrong guy to ask an opinion re. electric power: Have gotten rid of all things oily long ago already meself I have! Having said that- starting out in electrics is an expensive affair, but electrics is no black art in itsself. You can always convert the Cub later on if you like.

Keep the pictures coming!

Aug 01, 2006, 07:10 AM
DPlumpe's Avatar
About engines: The present 1/6 scale SIG Cub was a 1970s makeover of a Chuck Hollinger design in January, 1955, "Air Trails" magazine. Chuck used a Fox .19 in his. Although only 2 channels (rudder escapement and 2-speed throttle) with the 3-tube Babcock receiver and large pack of batteries, his weighed 4.5 lbs.

The present SIG kit is a bit beefed-up from Hollinger's design, but with modern equipment, I just don't see a new one being as heavy as his. So, unless you want to model the obscure FJ-3 fighter variant of the Cub, you really don't need an engine bigger than a .20 or .25 2-stroke or .26 or .30 4-stroke. If you put something as big as a .40 on it, make sure it'll idle REAL slow, or you'll have trouble landing it with the engine running. Beware that a 4-stroke engine requires the firewall to be moved back from the position on the plans.

These comments apply only to the SIG kit. Other kits I've seen are much heavier than the SIG, need more power, and have to fly faster to stay in the air.

Aug 01, 2006, 09:42 AM
Registered User
The SIG J-3 Cub history is a bit differnt in my memory. Berkeley kitted the Holliger design and issued a kit up to their sale in 1960. Then both Fox and SIG produced the same model. Fox used the original name and labeling, but an Arkansas address. SIG applied a whole new label, scrubbed off the title block, and added theirown. However, underneath it all was cut from same dies until about middle 1960's when they flooded the market and had unsold stock. It was again revised about 1969 with new parts and fiberglass cowl. It then got revised again around 1974 or 1975.

The A/C files OK with an original quality .35, but don't use one. The modern .25's are of equal thrust now, and have better throttles.

Aug 02, 2006, 10:51 PM
My hangar is,... "almost" full
No Step's Avatar
Sorry for the off topic question, but does anyone know of a Sig 1/5 scale (84"ws) build thread or review. I did a search but came up with nothing. I'm considering it,... or the Page 80" Piper Super Cruiser. Thanks.

Aug 03, 2006, 12:11 AM
Victim of gravity
No Step

There are huge build threads in the electric scale forum on both, the Page is current, the Sig (by Derek Woodward) you'll have to search for.
Aug 03, 2006, 09:57 AM
Registered User
Thanks For All The Input. I Think I Will Most Likely Buy A .29 2 Stroke. I Stopped By The Hobby Shop To Look At Options For Electric Flight, And Frankly I Don't Have Extra Money For Batteries. My Build Has Come To A Crawl As I Have Been Preparing For Certification Test For Usaf

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